A conventional oilfield pumping unit comprises a walking beam pivotally supported at a central portion thereof by a vertical tower, termed a Samson post, located adjacent the wellhead. A motor driven crank and connecting rod assembly drives the walking beam for vertical rocking reciprocation. A horsehead fixed to the end of the walking beam supports a cable sling attached to a hanger connected to a polished rod which extends vertically downwardly into the wellhead to a lift pump. The vertical rocking movement of the walking beam and horsehead causes the polished rod to reciprocate vertically of operate the pump to lift oil from the well. In such a conventional arrangement, the stroke, i.e. the extent of vertical reciprocation of the polished rod, is equal to the extent of vertical movement of the horsehead.
While oil field pumping units comprising conventional horseheads provide satisfactory performance under many operating conditions, they are unsatisfactory under circumstances where the pumping unit is underloaded, i.e. where the pumping unit is not moving the amount of fluid the unit is capable of moving because the polished rod does not provide sufficient stroke. This often occurs where considerable production volumes are to be pumped from shallow depths. In other words, the production rate of the conventional unit in such cases is substantially less than that of which it is capable owing to the relatively limited stroke of the polished rod.